Showing papers in "The Journal of Physical Chemistry in 1975"

Electron spin resonance studies of anisotropic rotational reorientation and slow tumbling in liquid and frozen media. III. Perdeuterated 2,2,6,6-tetramethyl-4-piperidone N-oxide and an analysis of fluctuating torques

Abstract: A detailed analysis of line shapes and relaxation is made for the perdeuterated 2,2,6,6-tetramethyl-4-piperidone N-oxide (PD-Tempone) nitroxide radical covering the whole range from fast motional narrowing ( TR - 10-12 sec) to the rigid limit (TR - 10-6 sec) in several deuterated solvents. The slow tumbling results show particularly good agreement with the slow tumbling theory of Freed, et al., for a reorientational model of moderate jumps (ca. 50° rrns), and essentially isotropic reorientation. It is shown that while such a model may not be readily distinguished, purely from its slow tumbling spectral predictions, from a free diffusion model including inertial effects, the latter model is incompatible with most other considerations. However, a simple analysis explicitly including the fluctuating (or random) torques indicates that more fundamental analysis of the dynamics may explain the slow tumbling results without necessarily involving substantial jumps. The spectral analysis is considerably enhanced by the increased resolution obtained from the use of the perdeuterated spin probe and deuterated solvents. Careful analysis of results at X-band and 35 GHz has shown that the nonsecular spectral densities exhibit significant deviations from a Debyelike spectral density yielding results very similar to those recently reported for the peroxylaminedisulfonate (PADS) radical. Related observations are discussed of apparent non-Debye-like spectral densities in the incipient slow tumbling spectra. These anomalies are also found to be amenable to a unified explanation in terms of the fluctuating torques. The analysis of much of the results in terms of a simple model yields an rms value for the fluctuating torques of ca. AT and TM - TR where TM is the relaxation time of the torques. The high-temperature electron-spin flip processes are consistent with a Hubbard-type spinrotational mechanism, but the low-temperature results may be due to spin-rotational relaxation from intramolecular motions. The simple analysis of spin-rotational relaxation in terms of relaxation of the fluctuating torques is found to yield equivalent predictions to conventional treatments when the estimated values of rms torque and TM are used.

Critical micelle concentration and the transition point for micellar size distribution

Abstract: The equation assumed by Ben-Naim and Stillinger for the free energy of micellization is used to demonstrate that, contrary to their assertion, the critical concentration corresponding to a transition in the shape of the micellar size distribution has a value close to the conventional cmc. The ratio cmc/C is shown to decrease sharply from the large value of 20 obtained by Ben-Naim and Stillinger, to a value of 1.55, if one of the parameters is assigned a physically more plausible value. This value accounts better for the cooperative-self-association of the surfactant molecules. 10 references.

Kinetics of the oxidation of sulfite by hydrogen peroxide in acidic solution

Abstract: The kinetics of the oxidation of sulfite by hydrogen peroxide has been investigated by stopped-flow spectrophotometry over the pH range 4-8. The rate law for the oxidation of sulfite in this pH range is -d[SO_2]_T/dt = k [H^+][H_2O_2][SO_2]_T([H^+]/([H^+] K_a_2)} + k'[HA][H_2O_2][SO_2]_T{[H^+]/([H^+] + K_a_2 where HA represents all possible proton donors in solution. The reaction probably proceeds via a nucleophilic displacement by H_2O_2 on HSO_3^- to form a peroxomonosulfurous acid intermediate which then undergoes a rate-determining rearrangement.

Role of ab initio calculations in elucidating properties of hydrated and ammoniated electrons

Abstract: The properties of solvated electrons are analyzed in terms of a self- consistent modified continuum model based on the techniques of ab initio molecular quantum mechanics. The model is semiclassical in spirit, employing the quantum mechanical density for the excess charge and the first solvation shell in conjunction with classical electrostatics, and is developed in a general form which can be straightforwardly applied to special cases of interest, such as the solvated mono- and dielectron complexes. The advantages and disadvantages of the technique are discussed in relation to other more empirical approaches. Computational results are presented for excess electrons (mono- and dielectrons) in water and ammonia, and the role of long-range polarization of the medium in localizing the excess charge is analyzed. The variationally determined ground states are characterized in terms of equilibrium solvation shell geometry (appreciable cavities are implied for both water and ammonia), solvation energy, photoionization energy, and charge distribution. The finding of negative spin densities at the first solvent shell protons underscores the importance of a many electron theoretical treatment. Preliminary results for excited states are also reported. The calculated results are compared with experimental and other theoretical data, and the sensitivity of the results to variousmore » features of the model is discussed. Particular attention is paid to the number of solvent molecules required to trap the excess electron. (auth)« less

Solvation time of the electron in polar liquids. Water and alcohols

Abstract: The formation of e$sup -$/sub sol/ from its precursor, e$sup -$/sub t/, was investigated in the temperature range from -65 to 60$sup 0$C using a stroboscopic pulse radiolysis (SPR) system. The rate of formation of e$sup -$/ sub sol/, measured at 600 nm, and rate of decay of e$sup -$/sub t/, measured at 1300 nm, are the same; the rate of decay of e$sup -$/sub t/ is slower by as much as a factor of 2 when measured at 1050 nm. At 20$sup 0$C the values of tau/sub sol/ are 10.7 +- 1, 23 +- 2, 34 +- 3, and 39 +- 5 psec for methanol, ethanol, 1- propanol, and 1-butanol, respectively. At 20$sup 0$C tau/sub sol/ is estimated to be less than 3 psec for H$sub 2$O and D$sub 2$O. In all of the alcohols tested there is a good correlation between tau/sub sol/ and the dielectric relaxation time, tau$sub 2$, for rotation of the molecules over a wide range of temperatures; this suggests that intermolecular hydrogen bonding does not limit the rate of electron solvation. (auth)